The present invention relates to breath analyzers in general and, in particular, to a new and useful method and apparatus for determining the alcohol concentration in the blood of a person to be tested by measuring the alcohol and humidity concentration in the person's breathing air.
The invention is based on the finding that there is a close relationship between the concentration of alcohol and humidity in air exhaled by a person. When a person to be tested tries to effect an exchange of air in his oral cavity and upper air passages by means of flat breathing to disguise the amount of alcohol which may be on his breath, this hardly affects the measuring result, since the ratio of alcohol to humidity is practically not influenced. When liquid evaporates in the oral pharyngeal space, the alcohol and the water are absorbed into the breathing air in a similar close ratio as in the lungs.
A known arrangement for determining alcohol concentration in the blood measures the alcohol in the breathing air at a time determined by a time control. This time is determined by a given time interval beginning within the expiration period. The rate of flow must not drop below a determined minimum rate during this time interval and the breathing air must always flow in an expiration direction. If these two conditions are not met, an error detector determines and indicates the invalidity of the measurement.
The object of the given time interval is to ensure that the test person has already exhaled the air from the oral cavity and the air pipe, and that the measuring instrument then measures the alcohol concentration of the breathing air from the alveoli of the lungs. The expiration of the given time interval is determined by the time at which a minimum breathing air volume of preferably at least 75% of the entire breathing air volume has been exhaled. An integrator can integrate the rate of flow of the breathing air during inspiration and expiration, and from there, determine the end of the time interval by the minimum breathing air volume. This device and method is independent of the physical structure of the person being tested. Thus, the method does not prevent measuring errors caused by a test person whose lungs are too large or who does not cooperate in the test. By means of deliberate flat inhalation, the test person can simulate a too low breathing capacity. The automatically established minimum breathing air volume during the test can then be practically all mixed air from the lungs, oral and pharyngeal space (See German Offenlegungschrift No. 24 28 352).
Another known method and the respective arrangement therefor are based on the consideration that the actual alcohol concentration in the breathing air can only be determined if that part of the exhaled air is analyzed for its alcohol content, which could establish equilibrium with the alcohol concentration of the blood in the alveoli of the lungs. The oscillating air from the oral and pharyngeal cavity and the mixed air must, therefore, be separated from the alveolar air in the measurement procedure. The method and the respective arrangement for this type of device resolves this problem by using an infrared measuring device which constantly measures the alcohol concentration during the sampling period. The change in time of the measured value is determined in a threshold value comparator, which represents the measurement for the rate of rise of the alcohol concentration.
A measured value is transmitted to the indicator only when the rate of rise drops below a given threshold value. This first condition results from the fact that the portion of the oscillating air from the oral and pharyngeal space diminishes with the drop of the rate of rise, and when it drops below the threshold value, only alveolar air is contained in the measuring channel of the arrangement. Another condition is that the measured value is transmitted when the rate of flow of the exhaling air is determined by a flow meter must be above a given value during a given time interval until the measured value is transmitted. This additional condition ensures the provided course of the measuring method. The alcohol concentration is measured by an infrared measuring device with a short response time connected into the breathing air current.
By meeting these three conditions, namely, determination of the change in time of the alcohol signal; measurement of the rate of exhalation compared to a given value, and minimum maintenance of this value over a given time, the device becomes voluminous and complicated and, moreover, requires corresponding supervision (See German Offenlegungschrift No. 26 10 578).